The present invention relates to a data transmission apparatus and, more particularly, to a data transmission apparatus in an ATM (Asynchronous Transfer Mode) communication system using a B-ISDN (Broadband Integrated Service Digital Network).
FIG. 6 shows an arrangement of a conventional data transmission apparatus of this type. This apparatus is disclosed in Japanese Patent Laid-Open No. 4-129351. Referring to FIG. 6, an input video signal supplied to the input terminal is input to a compressing/coding circuit 61 to be coded. In coding, the compressing/coding circuit 61 uses, for example, an entropy coding scheme, in which the compressing/coding rate varies with time.
The compressed/coded data with a variable rate, which is output from the compressing/coding circuit 61, is input to a cell generation circuit 62. In the cell generation circuit 62, cells are generated in units of pixel blocks, each consisting of a predetermined number of pixels. More specifically, the cell generation circuit 62 generates one or a plurality of cells as shown in FIG. 7C with respect to the compressed/coded data in FIG. 7B, which is obtained by compressing/coding the input video signal having a predetermined interval T as shown in FIG. 7A, i.e., a signal obtained by compressing/coding a pixel block consisting a predetermined number of pixels.
In executing such cell generation, since the data amount of the coded output of a given pixel block does not always coincide with an integer multiple of a cell, a remainder portion may be produced. So-called dummy data such as all "0"s is inserted in this remainder portion (see FIG. 7C). As described above, one or a plurality (an integer) of cells are generated for each pixel block and input to a variable-length cell block generation circuit 63.
The variable-length cell block generation circuit 63 generates variable-length cell blocks. Referring to FIG. 6, the cells generated by the cell generation circuit 62 from the video signal having the predetermined interval T as described FIGS. 7A to 7C are formed into a cell block. That is, one or a plurality of cells obtained in units of pixel blocks are formed into a cell block.
The variable-length cell block output obtained by the variable-length cell block generation circuit 63 is input to an error-correcting coding circuit 64 to be coded. Since each input to the error-correcting coding circuit 64 is a variable-length cell block, outputs like those shown in FIG. 8 are obtained. Referring to FIG. 8, reference symbols P1-1 to P3-2 denote check cells. In this case, two check cells are added to each cell block.
As described above, in order to perform error-correcting coding of a variable-length cell block, a fixed-length cell block having a sufficient length is assumed, and a portion corresponding to the difference between the fixed-length cell block and the actual variable-length cell block is assumed to be a cell (dummy cell) consisting of all-"0" data. In this manner, error correction is performed while the transmission delay is minimized, thereby allowing data transmission.
According to an ATM network in a B-ISDN, the transmission section of a transmission apparatus can recognize the error rate and the congestion state in the network on the basis of network management information represented by information obtained by an OAM (Operation Administration and Maintenance) function specified by the ITU recommendation I. 610 and the like.
In such a case, the transmission section can execute optimal control, depending on the state of the network, in accordance with the contents of data to be transmitted and the service form using the data. For example, when almost no congestion is recognized, data is transmitted without performing error-correcting coding to reduce redundant bits and transmission delay which occur in the process of error-correcting coding, thereby increasing the transmission efficiency or rate. In contrast to this, when the degree of congestion or the error rate increases, only error-correcting coding for bit errors is performed or strong error-correcting coding capable of coping with a cell loss is performed at the expense of the transmission efficiency depending on the degree of the congestion and the error rate.
In consideration of the above data transmission scheme, according to the conventional technique shown in FIG. 6, since error-correcting coding is always and permanently performed without recognizing the state of a network, optimal control cannot be performed by matching the transmission efficiency and rate with the state of the network.